The present invention relates in general to integrated circuits, and more particularly to sensing data stored in nonvolatile memory circuits.
Many wireless communications systems are using digital techniques to increase functionality and noise immunity while reducing cost. For example, cellular telephones and pagers receive radio frequency carrier signals modulated with digital data. A demodulator extracts the data and a digital signal processor executes a software program to convert the data to audio and display signals.
Portable wireless communications devices store the software program and other data in a nonvolatile memory circuit that operates from a low voltage battery, e.g., 1.8 volts, and has minimum current drain for extending the operating time between battery recharges. Data is stored in electrically erasable, floating gate storage transistors by applying high-voltage programming signals to induce a stored charge that controls the conduction of the transistor.
Prior art memory circuits read the stored data by driving a bitline with the storage transistor to produce a voltage swing that can be sensed and compared with a reference voltage to produce the data. However, bitline voltage transitions are slow and increase access time because of the need to discharge large bitline capacitances through the storage transistor. Such bitline discharging not only slows memory access, but also consumes power and reduces battery operating time.
Hence, there is a need for a memory circuit that can sense data stored in a memory cell while reducing bitline voltage swings in order to reduce power and improve performance.